In recent years, relaxor-PbTiO3 (relaxor-PT) single crystals have generated widespread interest for ultrasonic and actuation applications. Relaxor-PT crystals exhibit an electromechanical coupling close to the theoretical maximum (k33>0.9) and piezoelectric voltage-strain coefficients up to a factor of five higher than traditional PZT ceramics. Originally this class of materials included only (1−x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT) and Pb(Zn1/3Nb2/3)O3—PbTiO3 (PZN-PT), but a number of new relaxor-PT crystals have been reported that exhibit higher Curie temperatures and other improved properties. Consequently, relaxor-PT crystals show promise for replacing traditional PZT ceramics in a number of important areas in the coming years. The most widely-available relaxor-PT material is PMN-PT. Much work has been devoted to characterizing the crystallographic phase diagram (shown in FIG. 1) of PMN-PT between room temperature and the Curie temperature over a wide compositional range.
Like other relaxor-PT single crystals, PMN-PT exhibits its strongest piezoelectric effects when the composition is situated near a morphotropic phase boundary (MPB) between two distinct crystalline structures. As can be seen in FIG. 1, PMN-PT with PT concentration below 30%, forms a rhombohedral (3 m) piezoelectric single crystal, while at PT concentrations larger than 35% the crystal exhibits the same tetragonal (4 mm) symmetry of pure PT. It has been suggested that the polarization of the crystal rotates through intermediate monoclinic phases, shown in FIG. 1 which helps to explain the high susceptibility and piezoelectric coefficients of these materials. The room temperature zero-field phase of PMN-PT in the composition range from 30% to 35% is a monoclinic “C” phase denoted MC. The different phases correspond to different directions of the ferroelectric dipole moment of the crystal unit cell relative to the crystalline axes. When heated past a specific temperature, denoted TRT or TMcT, the dipole moment of the crystal aligns with one of its pseudocubic axes, creating a crystal with tetragonal symmetry. Further heating past the Curie temperature depoles the crystal, destroying the ferroelectric dipole moment altogether, and the crystal assumes cubic symmetry. All relaxor-PT crystals reported thus far exhibit both of these phase transitions temperature within some compositional range, but the first transition at TRT is not observed in traditional piezoelectric ceramics like PZT or in single-domain crystalline piezoelectrics like lithium niobate.
Despite the considerable research that has been devoted to the equilibrium phase diagram of relaxor-PT materials, there has been relatively little work on the non-equilibrium properties of the materials. The crystal structure of these materials transitions between several different possible phases with very little compositional change. As such, the material experiences relatively unstable equilibrium states. The different crystal phases have been shown to exhibit vastly different electrical, mechanical, piezoelectric, pyroelectric and optical properties. The crystal structure of these materials is dependent not only on chemical composition and temperature, but also on applied bias electric field, cooling rate, applied stress and load histories of these factors. In known relaxor-based piezoelectric materials, a single crystal structure phase is present in the material at any one time at room temperature. One study has shown that cooling rate can have an effect on domain size of the resulting crystal but did not demonstrate a crystal structure change. Interestingly, the crystal structure phase transitions and domain size that are associated with bias electric field, heating and cooling rates and applied stress appear to be completely reversible.
There remains a need to develop new piezoelectric materials and methods for control of their properties.